1. Field of the Invention
The present invention relates to the preparation of improved polishing compositions, to the resultant compositions themselves, and to the use of such compositions for the polishing of glass or like materials. More especially, this invention relates to the continuous preparation of polishing compositions comprising the rare earths.
2. Description of the Prior Art
Among the polishing compositions currently used in the glass industry, those based upon the rare earths, and in particular cerium, typically perform the best. A variety of processes for the manufacture or preparation of such compositions are also known to this art. Thus, it has been proposed, according to published German Patent Application No. 2,426,110, to precipitate an aqueous solution of the rare earth sulfates by means of sodium hydroxide, in the presence of molochite, and to filter off, wash, dry and calcine the resulting product in order to obtain the desired polishing composition. A process of this type, though, is incapable of providing wholly satisfactory polishing compositions from a total effectiveness standpoint, in particular because of the heterogeneity, the undefined structure and the irreproducibility of the resultant compositions. These disadvantages notably arise from the conditions under which the process is carried out, and according to which conditions the concentration of the reactants changes during the reaction and it is thus impossible to repeatedly prepare any given composition in a completely reproducible manner; such disadvantages also ensue by reason of the existence of foreign compounds (molochite) and the presence of sulfates, the precipitation of which by means of sodium hydroxide giving rise to complex mixtures of products, such as double sulfates, hydroxysulfates and hydroxides, the amount and nature of which can vary during the reaction.
It has also been proposed [Chemical Abstracts, 80, 51688, (1974)] to prepare cerium-based polishing compositions by precipitating ceric hydroxide, by means of ammonia, from a rare earth nitrate solution in which the cerium has been preliminarily oxidized. A process of this type requires an additional purification step entailing recrystallization via cerium/ammonium nitrate, and recovering the calcined oxide by means of a dilute acid; the resultant compositions are inadequate from an effective polishing point of view. Furthermore, such compositions also do not possess satisfactory properties of homogeneity, structure and reproducibility.